Inorganic cements exhibit characteristic properties of setting and hardening when mixed with water to form a paste. They are capable of joining rigid solid masses into coherent structures. Inorganic cements can be divided into hydraulic and nonhydraulic types according to the way in which they set and harden. Hydraulic cements are capable of setting and hardening under water, whereas nonhydraulic cements harden in air and cannot be used under water. See Z. D. Jastrebski, The Nature and Properties of Engineering Materials, 2d. Ed., John Wiley & Sons, New York (1977) at 356, the disclosure of which is incorporated by reference herein.
The most widely-used hydraulic cement is so-called Portland cement, which is obtained by heating an intimate mixture, composed mainly of calcareous and argillaceous materials, or other silica, alumina, and iron-oxide bearing materials, at a clinkering temperature of about 1400.degree. C. The partially sintered material, called clinker, is then ground to a very fine powder. After mixing with water, a hardened Portland cement paste is a calcium-silicate hydrate (C-S-H) which, like other gels, contains a network of capillary pores and gel pores. The total porosity of a typical hardened Portland cement paste is about 30-40% by volume, having a very wide pore-size distribution ranging from 10-0.002 .mu.m in diameter. The gel porosity, consisting of very small pores, is about 26%, with the remaining porosity due to capillary network. See Z. D. Jastrebski, supra, at 356-61.
Cement formulations may also contain additional additives. For example, small amounts of calcium sulfate in the form of gypsum or anhydrite may be added during grinding of the raw materials to control the setting time and enhance strength development of Portland cement. Cements are sometimes impregnated with liquid organic monomers or liquid sulfur and polymerized to produce polymer-impregnated concrete. See 5 Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed., John Wiley & Sons, New York (1978) at 163, the disclosure of which is incorporated by reference herein. Other additives include water reducers, plasticizers, air entrainment, microsilicates, and the like.
Several methods of increasing the strength of cement are known. For example, high-strength Portland cement pastes can be produced using specially ground cement with the assistance of surfactant grinding aids to produce surface areas ranging from 0.6 to 0.9 m.sup.2 /g. When mixed with water and plasticizing agents, the hardened pastes show decreased porosity and compressive strength which is about twice the strength of cement pastes produced by conventional methods. See Z. D. Jastrebski, supra, at 361.
High strength cements are also obtained by hot pressing conventional cement pastes under pressure of 196 to 392 MPa and at a temperature of 150.degree. C. The hardened cement paste thus obtained exhibits nearly zero porosity and shows compression strength, tensile strength, and shear strength values which are about four times greater than the strengths of cement pastes produced by conventional methods. See Z. D. Jastrebski, supra, at 361.
Although known methods of improving the strength of hardened cement can be quite effective, they necessitate specialized, time-consuming processing steps such as grinding and hot pressing. In addition, the cement compositions themselves may have to be modified by the addition of expensive specialty chemicals.
Therefore, there is a need for easily, inexpensively-obtained concrete compositions having higher strength and density, and lesser porosity, than or concrete compositions currently available.
There is also a need for simple, inexpensive methods of improving the strength and other desired properties of concrete.
Accordingly, it is an object of the present invention to provide a high-strength concrete composition having greater strength and density, and lesser porosity, than or concrete compositions previously known.
It is also an object of the present invention to provide a method of manufacturing a high-strength shaped article.